The present invention relates to an improved method of drying ceramic honeycomb substrates such as those utilized in catalytic converter, and in particular to controlling the shrinkage during the drying process of such ceramic honeycomb substrates.
Differential or non-uniform shrinkage during the drying process of ceramic honeycomb substrates has long been a problem in the art. It is readily known, that the skin or outer portion of the ceramic honeycomb substrates dries at a faster rate than the core or interior portion. This is because at the skin, the rate of evaporation of water molecules is faster than the rate of water molecules migration from the interior portion to the outer portion of the substrate. In essence at the surface water molecules evaporate faster than they are replenished. As a result there is more shrinkage at the skin than at the core of the substrate, which in turn creates warping or distortion and leads to the development of harmful stresses which may result in cracking of the part during subsequent firing processes.
Accordingly, there is a need for a process for controlling differential shrinkage during drying in honeycomb substrates, in order to minimize the development of harmful stresses within the structure of ceramic honeycomb substrates.
The invention is directed to a method of controlling the drying shrinkage in ceramic honeycomb substrates of the type used in automotive catalytic converters. Such honeycomb bodies typically are made of cordierite and include a structure having thin interconnecting porous walls which form parallel cell channels longitudinally extending between the end faces of the structure, as disclosed in U.S. Pat. Nos. 2,884,091, 2,952,333, and 3,242,649. Honeycomb ware is typically manufactured by extruding or fabricating ceramic material into logs, followed by cutting, drying and firing.
The method of the present invention which provides the improved drying of green or wet honeycomb substrates by controlling the drying shrinkage comprises drying the honeycomb substrate with a vapor-insulating guard substantially disposed about the outer surface of green honeycomb substrate. The vapor-insulating guard acts to retard the rate of evaporation of water molecules at the skin or outer portion, thereby effecting an equilibrium between the rate of evaporation of water molecules at the skin and the rate of migration of the water molecules from the core or inner portion of the substrate. In order to maintain this evaporation-migration equilibrium the vapor-insulating guard must be disposed about the green honeycomb substrate throughout the drying process.
The vapor-insulating guard can be made of any material that acts as a barrier to the evaporation of water molecules at the skin or outer portion of the ceramic honeycomb substrate. Preferably, the vapor-insulating guard is made of plastic, and more preferably, a thin plastic sheet such as commercially available SaranWrap(trademark) plastic film. Other suitable choices include Mylar(trademark) plastic sheets, and plastic tubing made of for example Lexan(trademark).